This invention relates to shaping of heat softened sheet material, and in particular to shaping heat softened glass sheets between an upper mold and a lower flexible ring mold.
Shaped and tempered glass sheets are widely used as windows in vehicles such as automobiles and the like. To fabricate these windows, flat glass sheets must be shaped to precisely defined curvatures dictated by the shape and outline of the window frame openings in the vehicle. It is important that the windows meet stringent optical requirements and be free of optical defects that would tend to interfere with clear viewing through the window. Any distortion in the shaping members that engage the heat softened sheet is replicated in the major surface of the sheet and result in an optically deficient surface on the shaped sheet.
Commercial production of shaped glass sheets commonly includes heating flat glass sheets to their softening temperature, shaping the heated sheets to a desired curvature and then cooling the shaped glass sheets in a controlled manner. In particular, the glass sheets are serially conveyed to a tunnel type furnace and heated to their heat deformation temperature and thereafter conveyed into a shaping station where the heat softened sheets are sandwiched between a pair of vertically, aligned upper and lower shaping molds. After shaping, the molds separate with the shaped glass sheet remaining engaged by the upper mold by vacuum. A transfer ring having an outline shape and conforming to the desired curvature of the glass sheet slightly inboard of its perimeter moves beneath the upper mold which thereafter releases the vacuum and deposits the shaped glass on the ring. The ring then transfers the shaped glass sheet into the cooling station for tempering.
The lower mold in such sheet shaping arrangement is generally positioned below the conveying rolls in the shaping station and may include a flexible shaping ring as disclosed in U.S. Pat. No. 4,830,650 to Kelly. During shaping, the lower mold moves upward and lifts the glass sheet off the conveying rolls and into engagement with the upper mold. As the flexible ring lifts the glass sheet and presses it against the upper mold, the flexible ring conforms to corresponding portions of the upper mold. It has been found that during the initial rapid upper movement of the lower ring, the ring tends to deform downward prior to the glass engaging the upper mold, due in part to the acceleration of the ring and the weight of the supported glass sheet. The shape assumed by the glass may not correspond to the final desired shape. This may result in some sliding of the glass sheet and marking as it is lifted by the ring and pressed against the upper mold.
It would be advantageous to provide an arrangement that limits the flexible ring deflection during lifting of the glass but does not interfere with the pressing and flexing operation of the ring.